1. Field of the Invention
The present invention relates to a magnetic resonance imaging system. More particularly, the invention relates to a technique of performing high-speed imaging while suppressing artifacts caused by body movement.
2. Background Art
The magnetic resonance imaging system is an apparatus that transmits a measuring object placed in a static magnetic field with a high-frequency electromagnetic field (RF pulse) of a specific frequency and induces a nuclear magnetic resonance phenomenon to acquire physicochemical information of the measuring object. Magnetic resonance imaging (MRI) which is currently in wide-spread use mainly uses a nuclear magnetic resonance phenomenon of hydrogen nuclei in water molecules and images differences in the density of hydrogen nucleus or relaxation time or the like which varies from one tissue to another. After and/or with transmitting RF pulses, the MRI applies a slice gradient magnetic field, phase encoding gradient magnetic field and reading gradient magnetic field to add spatial information necessary to reconstruct the image to a magnetic resonance signal (echo).
When spatial information is added by repeatedly adding phase encoding gradient magnetic fields of different intensities before echo formation, if there is movement in the measuring object (body movement), a phase error is likely to be contaminated in the direction in which the phase encoding gradient magnetic field is applied (phase encoding direction) and artifact flowing in the phase encoding direction (body movement artifact) is prone to occur in the image to be reconstructed from an echo thereof. In diffusion weighted imaging, which measures the intensity of molecular diffusion of water, this body movement artifact becomes conspicuous. This is because the diffusion gradient magnetic field is intended to produce signal attenuation in accordance with the amount of diffusion coefficient and if body movement occurs while this diffusion gradient magnetic field is being applied, a large phase error occurs.
Imaging methods such as an Echo Planar Imaging (EPI) method and a line scan method, which do not repeatedly apply the above described phase encoding gradient magnetic field, have little influence of body movement artifact. However, the EPI method applies a gradient magnetic field called an “oscillating gradient magnetic field” while acquiring echoes by switching the gradient magnetic field at a high speed. This requires a high performance gradient magnetic field system and causes the image to be distorted due to the influence of an inhomogeneous static magnetic field.
The line scan method selects and excites one slice plane by a first RF pulse, selects and inverts another slice plane which crosses the selected and excited slice plane by a second RF pulse (inversion pulse) and measures an echo from a linear area in which the two slice planes cross each other. The linear area is sequentially moved, measurement is repeated and the area necessary to reconstruct the image is measured. The line scan method does not use any phase encoding gradient magnetic field, and is therefore used for diffusion weighted imaging when measuring regions susceptible to body movement (e.g., see Non-Patent Document 1). The technique is also available to be combined to chemical shift imaging which separates and images each metabolite distribution using chemical shift of a metabolite and diffusion weighted imaging (EPSI: Echo Planar Spectroscopic Imaging) with the line scan method (e.g., Patent Document 1, Non-Patent Documents 2 and 3).
Some methods have been proposed to shorten the measurement time by the line scan method such as a method of moving a crossing area in a direction inclined with respect to both of the two slice planes and shortening a repetition time (see, Patent Document 2, for example) and a method of applying a first excitation pulse twice, using two echoes thereby obtained and simultaneously acquiring two slices (see, Non-Patent Document 4, for example). Furthermore, in order to improve the quality of a reconstructed image obtained by the line scan method, a method changing the amplitude of a crusher gradient magnetic field applied so as to prevent a resonant signal in a previously excited crossing area to be contaminated therein when scanning the crossing area has been proposed (see, Patent Document 3, for example). Furthermore, in order to increase spatial resolution of an image obtained by the line scan method, a method of acquiring crossing areas superimposed on each other and meticulously imaging the inside of the crossing area by high-speed imaging has been proposed (see, Patent Document 4, for example). A method of detecting large body movement and remeasuring it to prevent losses of images which may occur when large body movement takes place has been proposed (see, Patent Document 5, for example).
On the other hand, among general measurement methods of MRI, a method is proposed which uses differences in sensitivity distribution among a plurality of RF coils to shorten the measurement time (see, Non-Patent Documents 5 and 6, for example). The method performs measurement by reducing the number of times a phase encoding gradient magnetic field applied, and obtained folded image in the phase encoding direction is unfolded using differences in sensitivity distribution among the respective RF coils. Since the number of times phase encoding applied decreases, the measurement time is shortened. Furthermore, shortening of the measurement time also reduces the probability that body movement occur during the measurement time which leads suppression of body movement artifacts.    [Patent Document 1] JP Patent No. 3588690    [Patent Document 2] JP Patent Publication (Kokai) No. 63-105749A (1988)    [Patent Document 3] JP Patent No. 3342853    [Patent Document 4] JP Patent No. 3415754    [Patent Document 5] JP Patent No. 3884283    [Non-Patent Document 1] Gudbjartsson H et al., Line scan diffusion imaging. Magnetic Resonance in Medicine, vol. 36, pp. 509-519 (1996)    [Non-Patent Document 2] Bito Y et al., Echo-planar diffusion spectroscopic imaging: reduction of motion artifacts using line-scan technique. Proceedings of International Society for Magnetic Resonance in Medicine, p. 1235 (1998)    [Non-Patent Document 3] Oshio K et al., Line scan echo planar spectroscopic imaging. Magnetic Resonance in Medicine, vol. 44, pp. 521-524 (2000)    [Non-Patent Document 4] Gudbjartsson H et al., Double line scan diffusion imaging. Magnetic Resonance in Medicine, vol. 38, pp. 101-109 (1997)    [Non-Patent Document 5] Sodickson D K et al., Simultaneous acquisition of spatial harmonics (SMASH): fast imaging with radiofrequency coil arrays. Magnetic Resonance in Medicine, vol. 38, pp. 591-603 (1997)    [Non-Patent Document 6] Pruessmann K P et al., SENSE: Sensitivity encoding for fast MRI. Magnetic Resonance in Medicine, vol. 42, pp. 952-962 (1999)